Elecraft KAT500 Serial Command Reference - Manual (Page 15)

SL (Enable or disable Sleep when Idle, GET and SET)
GET format: SL;
SET/RESPONSE format: SL0; (disable) or SL1; (enable)
SL1; causes the KAT500 microcontroller (MCU) to enter a low power and electrically quiet “SLEEP” mode
when switched off or when idle. The 16 MHz crystal oscillator is turned off while sleeping.
The sleeping microcontroller can be awakened by:
1) Front panel button press,
2) Change to any of the accessory connector BAND lines,
3) High to low transition of the accessory connector AUXBUS line,
4) High to low transition of the “start tune” line of the rear-panel TUNE connector,
5) High to low transition (a start bit) on the RS-232 serial port, or
6) Transmit-level RF.
While the microcontroller can be awakened by characters arriving on the RS-232 serial port, it takes
about 100 milliseconds for the crystal oscillator to reach operating speed, and characters arriving in that
period may be lost. Computer programs should send a series of single semicolons at approximately 100
millisecond intervals until the KAT500 returns semicolon responses. It normally takes two or three
semicolons. Once the microcontroller is awakened by serial port communications, it stays awake until
no characters arrive for a few seconds.
The microcontroller is deemed “idle” when a short time period has elapsed without front panel button
presses or changes to any of the rear panel inputs (band, tune start, AUXBUS, serial port
communications, and RF transmission). The microcontroller is not “idle” while tuning (even if RF ceases)
or part of a serial command has been received.
Sleep when Idle was introduced in firmware version 01.38. Install KAT500 Utility version 1.13.4.2 or later
before enabling Sleep when Idle. It has been changed to send a few “wakeup” semicolons in each of its
communication sequences with the KAT500, and adds an Edit Configuration dialog Idle Sleep tab.
If you use the KAT500 Utility Command Tester, it might be necessary to enter a few semicolons before a
command is accepted. It is sometimes more convenient to turn off Sleep when Idle (SL0;) when testing
PC software or commands from the Command Tester window.
SM (Save Memory, SET only)
SET format: SM; or SM fffff; where fffff is a frequency in kHz.
SM saves the current tuner settings (antenna, side, inductor, capacitor, and bypass VSWR) in the tuner
memory for the indicated (or most recent transmit) frequency.